1. Field of the invention:
This relates to a flow sensor that detects not only the rate of flow but also the direction of flow of a fluid such as gas or liquid.
2. Description of the prior art:
As thermal flow sensors that detect the direction of flow of a fluid, semiconductor type flow sensors have been proposed in Japanese Laid-Open Patent Publication No. 60-247171. Since silicon is used for a substrate of the flow sensors, the flow sensors have an high thermal conductivity and the difference in temperature within the sensor chip that arises from the flow of gas is in the range of around 1/100.degree. to 1/10.degree. C. (Electronic Materials, No. 12, pp. 38-43 (1983)). The said difference in temperature can be determined by detecting the difference in temperature between a pair of temperature sensing transistors with high sensitivity that constitute a bridge circuit in each flow sensor. Although these flow sensors can be mass-produced by a semiconductor processing technique using silicon, they are disadvantageous in that the temperature characteristics vary between the flow sensors. Moreover, it is impossible to set the temperatures of these flow sensors at a high level.
In order to solve the above-mentioned problems, a thermal flow sensor that can be mass-produced with uniform sensor-characteristics and that attains stabilized operation even at a high temperature has been proposed in Japanese Patent Application No. 61-232885 "A FLOW SENSOR" that was filed on Sept. 29, 1986 in the name of this applicant, as shown in FIGS. 8A and 8B, in which a pair of fluid temperature sensing resistors 4a and 4b are disposed on a heat-insulating substrate 1 in such a manner that the fluid temperature sensing resistors 4a and 4b are symmetrical about a heating resistor 3a positioned in the center of the said substrate 1. The substrate 1 has a thermal conductivity of 2 w/m.k or less. The temperature of the heating resistor 3a is controlled so that the difference in temperature between the fluid and the heating resistor 3a can be maintained at a fixed level. From current that flows through the heating resistor 3a (the said current changing with a change in the flow rate of the fluid) or from the electrical potential or voltage that changes that a change in the said current flowing through the heating resistor 3a, the flow rate of the fluid is measured. At the same time, the difference in temperature arising between the two fluid-temperature sensing resistors 4a and 4b is measured as the difference in current or in voltage therebetween, from which the direction of flow of the fluid can be detected. A coated film 2 is deposited therebetween so as to strongly adhere the resistors 3a, 4a and 4b to the substrate 1. A monitor 3b that monitors the temperature of the heating resistor 3a is disposed in the vicinity of the heating resistor 3a. FIG. 9 shows the relationship between the direction of flow of the fluid, .theta., and the difference in temperature between the fluid temperature sensing resistors 4a and 4b, wherein the said difference in temperature is represented by the difference in voltage, V.sub.D, that is determined by the use of a bridge circuit. This flow sensor is disadvantageous in that two values of an angle .theta. indicating the direction of flow of the fluid for one value of the difference in voltage (i.e., the output voltage), V.sub.D, are obtained. Moreover, since the output voltage V.sub.D is influenced by the flow rate of the fluid, calibration of the output voltage V.sub.D is necessary.